Add basic mockgcp test cases for CloudSQL

[jasonvigil]

These test cases specify the "required" fields for postgres, mysql, and sqlserver and include updates for the non-immutable fields.

• [ ] Run make ready-pr to ensure this PR is ready for review.
• [ ] Perform necessary E2E testing for changed resources.

[yuwenma]

Looks good! Have some minor comments.

[google-oss-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: yuwenma

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[yuwenma]

/lgtm

[yuwenma]

/hold cancel